How DDoS Attacks Work

DDoS is an advanced version of the DoS attack. Like DoS, DDoS tries to deny access to services running on a system by sending packets to the destination system in a way that the destination system can't handle. The key of a DDoS attack is that it relays attacks from many different hosts (which must first be compromised), rather than from a single host like DoS. DDoS is a large-scale, coordinated attack on a victim system.

Hacking Tools

Trinoo is a tool that sends User Datagram Protocol (UDP) traffic to create a DDoS attack. The Trinoo master is a system used to launch a DoS attack against one or more target systems. The master instructs agent processes (called daemons) on previously compromised systems (secondary victims) to attack one or more IP addresses. This attack occurs for a specified period of time. The Trinoo agent or daemon is installed on a system that suffers from a buffer overflow vulnerability. WinTrinoo is a Windows version of Trinoo and has the same functionality as Trinoo.

Shaft is a derivative of the Trinoo tool that uses UDP communication between masters and agents. Shaft provides statistics on the flood attack that attackers can use to know when the victim system is shut down; Shaft provides UDP, ICMP, and TCP flooding attack options.

Tribal Flood Network (TFN) allows an attacker to use both bandwidth-depletion and resource-depletion attacks. TFN does UDP and ICMP flooding as well as TCP SYN and smurf attacks. TFN2K is based on TFN, with features designed specifically to make TFN2K traffic difficult to recognize and filter. It remotely executes commands, hides the source of the attack using IP address spoofing, and uses multiple transport protocols (including UDP, TCP, and ICMP).

Stacheldraht is similar to TFN and includes ICMP flood, UDP flood, and TCP SYN attack options. It also provides a secure telnet connection (using symmetric key encryption) between the attacker and the agent systems (secondary victims). This prevents system administrators from intercepting and identifying this traffic.

Mstream uses spoofed TCP packets with the ACK flag set to attack a target. It consists of a handler and an agent portion, but access to the handler is password protected.

The services under attack are those of the primary victim; the compromised systems used to launch the attack are secondary victims. These compromised systems, which send the DDoS to the primary victim, are sometimes called zombies or BOTs. They're usually compromised through another attack and then used to launch an attack on the primary victim at a certain time or under certain conditions. It can be difficult to track the source of the attacks because they originate from several IP addresses.

Normally, DDoS consists of three parts:

Master/handler

Slave/secondary victim/zombie/agent/BOT/BOTNET

Victim/primary victim

The master is the attack launcher. A slave is a host that is compromised by and controlled by the master. The victim is the target system. The master directs the slaves to launch the attack on the victim system. See Figure 1.

Figure 1: Master and Slaves in a DDoS Attack

DDoS is done in two phases. In the intrusion phase, the hacker compromises weak systems in different networks around the world and installs DDoS tools on those compromised slave systems. In the DDoS attack phase, the slave systems are triggered to cause them to attack the primary victim. See Figure 2.